Sheet conveying apparatus, and image forming apparatus

ABSTRACT

In one embodiment, a sheet conveying apparatus has a first conveying guide and a second conveying guide which is opposite to the first conveying guide. The sheet conveying apparatus further has at least one first concave portion. The first concave portion is provided in the first conveying guide to be recessed in a direction to separate from one surface of a sheet.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2015-093145, filed on Apr. 30, 2015, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a sheet conveying apparatus to convey a sheet along a conveying guide, and an image forming apparatus having this sheet conveying apparatus.

BACKGROUND

Conventionally, as a sheet conveying apparatus, an apparatus in which a sound absorbing material is provided on an inner surface of a conveying guide, in order to reduce noise at the time of conveying a sheet is known. A concave portion so as to increase a surface area of the sound absorbing material is provided, on a surface of the sound absorbing material.

For example, in an image forming apparatus such as a copying machine and a printer, when a sheet is conveyed along a conveying guide, a friction sound is generated due to sliding contact between a surface of the conveying guide and the sheet. In addition, a collision sound is also generated between a sheet to be conveyed and a conveying roller. In addition, a sound when a sheet to be conveyed on a curved conveying path warps, and a sound when an edge of the warped sheet abuts against a conveying guide, and so on are also generated. In addition, a sound of a motor to drive a conveying roller and a sound of a fan, and so on are generated.

In order to check a frequency characteristic of a sound generated from a copying machine, a microphone was installed at a position 1 m away from the front of the copying machine, and an operation sound was measured when a document was fed via an automatic document feeder of the copying machine. An example of the result obtained by performing a frequency analysis of the measured operation sound is shown in FIG. 17. In the graph of FIG. 17, a vertical axis is a sound pressure level, and a horizontal axis is a frequency. When FIG. 17 is seen, it is understood that the operation sound at the time of conveying a document via the automatic document feeder has a wide frequency band. That is, since noise caused by the conveyance of a sheet is a sound in which the above-described various sounds are mixed, its frequency band also becomes wide.

For this reason, when a sound absorbing material is simply provided on a conveying guide, as in the above-described conventional image forming apparatus, though a sound of a specific frequency band can properly be absorbed, but on the other hand, the effect of absorbing a sound of the other frequency band can hardly be expected. FIG. 18 shows a sound absorption characteristic when a thickness of urethane foam that is a representative sound absorbing material is changed. A vertical axis of the graph of FIG. 18 is a vertical incident method sound absorptivity (an absorptivity measured by a vertical incident method), and a horizontal axis is a frequency. According to FIG. 18, in any cases of thicknesses, a sound absorptivity shows a peak at about 2000 Hz. According to FIG. 18, it is understood that a sound absorption effect of a sound of a frequency other than about 2000 Hz is small. That is, when a sound absorbing material was only provided on the conveying guide, a sound contained in a certain specific frequency band could be reduced, but it was difficult to reduce the whole noises. In addition, a sound absorbing material is used, and thereby the number of components increases, and the manufacturing cost of the apparatus becomes high.

Accordingly, the development of a sheet conveying apparatus which can reduce a sound of a wide frequency band with an inexpensive configuration has been desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an image forming apparatus according to an embodiment.

FIG. 2 is a schematic diagram showing a sheet conveying apparatus according to an embodiment.

FIG. 3 is an enlarged diagram showing the sound deadening structure provided in the document conveying path of FIG. 2.

FIG. 4 is a sectional diagram along F4-F4 of FIG. 2.

FIG. 5 is a sectional diagram along F5-F5 of FIG. 2.

FIG. 6 is a sectional diagram showing a modification of FIG. 5.

FIG. 7 is a sectional diagram showing a modification of FIG. 5.

FIG. 8 is a sectional diagram showing a modification of FIG. 5.

FIG. 9 is a sectional diagram showing a modification of FIG. 5.

FIG. 10 is a schematic diagram showing an example of a testing device for describing an attenuation amount of a sound by the sound deadening structure of FIG. 3.

FIG. 11 is a graph showing the relation between an attenuation amount and L/λ described using the device of FIG. 10.

FIG. 12 is a diagram showing an attenuation amount distribution when a length L of the absorption chamber of FIG. 3 is set to 0.01 m.

FIG. 13 is a diagram showing an attenuation amount distribution when the length L of the absorption chamber of FIG. 3 is set to 0.02 m.

FIG. 14 is a diagram showing an attenuation amount distribution when the length L of the absorption chamber of FIG. 3 is set to 0.03 m.

FIG. 15 is a graph showing the change of an attenuation amount when the length L of the sound absorption chamber of FIG. 3 is changed.

FIG. 16 is a schematic diagram showing a modification in which a protrusion is provided on the upper guide plate of FIG. 6.

FIG. 17 is a graph showing a result obtained by performing a frequency analysis of an operation sound of a conventional copying machine.

FIG. 18 is a graph showing a characteristic of a conventional sound absorbing material.

DETAILED DESCRIPTION

According to one embodiment, a sheet conveying apparatus has a first conveying guide, a second conveying guide, and a first concave portion. The first conveying guide is arranged at one surface side of a sheet to be conveyed. The second conveying guide is arranged opposite to the first conveying guide, and at the other surface side of the sheet. The first concave portion is provided in the first conveying guide to be recessed in a direction to separate from the one surface of the sheet, by at least one. The first concave portion extends a conveying space defined between the first conveying guide and the second conveying guide, to cause an acoustic impedance to be changed, for example, and thereby reduces a sound generated at the time of conveying the sheet.

Hereinafter, further embodiments will be described with reference to the drawings. In the drawings, the same symbols indicate the same or similar portions. FIG. 1 is a schematic diagram showing a copying machine 100 according to an embodiment of an image forming apparatus. The copying machine 100 has an image forming device 10 at an approximately central portion in a chassis 101. The image forming device 10 forms an image, based on a document image read by a scanner 7 described later, or a document image obtained from the outside. For example, the image forming device 10 has a yellow image forming unit 10Y, a magenta image forming unit 10M, a cyan image forming unit 10C, a black image forming unit 10B. The image forming units 10Y, 10M, 10C, 10B of the four colors are arranged together separately from each other in the horizontal direction. Each of the image forming units 10Y, 10M, 1C, 10B has a photoreceptor that is an image carrier. The image forming unit 10Y forms a toner image of yellow color on the photoreceptor, using a toner of yellow color. The image forming unit 10M forms a toner image of magenta color on the photoreceptor, using a toner of magenta color. The image forming unit 1C forms a toner image of cyan color on the photoreceptor, using a toner of cyan color. The image forming unit 10B forms a toner image of black color on the photoreceptor, using a toner of black color. Hereinafter, the above-described toner image is simply called an image.

The copying machine 100 further has a transfer device, a sheet feeding device, a sheet conveying device, a fixing device, a sheet discharge device, and a reading device. The transfer device has a transfer belt 1, and a transfer roller 3. The transfer belt 1 is an endless belt which is arranged along the respective photoreceptors of the image forming units 10Y, 10M, 10C, 10B. The above-described images of the four colors, for example, formed by the image forming units 10Y, 10M, 10C, 10B are transferred from the above-described respective photoreceptors to the transfer belt 1. The transfer belt 1 runs while carrying the above-described image. The sheet feeding device has a sheet feeding cassette 2 and a pickup roller 2 a. The sheet feeding cassette 2 houses a sheet P (sheet) on which the above-described image is to be formed. The pickup roller 2 a picks up the sheet P from the sheet feeding cassette 2, and feeds the sheet P to a main conveying path 8 (a first conveying path) of the sheet conveying device. The sheet conveying device has the above-described main conveying path 8. The main conveying path 8 is a conveying path of the sheet P, from the above-described sheet feeding device to the conveying device, via the transfer roller 3 of the transfer device and the fixing device. The main conveying path 8 conveys the sheet P to be fed by the pickup roller 2 a to a position of the transfer roller 3 of the above-described transfer device. The transfer roller 3 is arranged opposite to the transfer belt 1. The transfer roller 3 transfers the above-described image carried by the transfer belt 1, from the transfer belt 1 to the sheet P. The main conveying path 8 conveys the sheet P to which the above-described image(s) has been transferred to the fixing device. The fixing device is a fixing unit 4 including a heat roller not shown, for example. The fixing unit 4 fixes the image to one surface of the sheet P. The image is finally formed on the sheet P, with the above-described processes by the above-described respective devices. The main conveying path 8 conveys the sheet P formed with the image to the sheet discharge device. The sheet discharge device has a sheet discharge tray 5 and a sheet discharge roller 5 a. The sheet discharge roller 5 a discharges the sheet P formed with the image to the sheet discharge tray 5. The sheet discharge tray 5 holds the discharged sheet P. The reading device has a document table 6 and the scanner 7. The document table 6 supports a document D to be loaded on the surface thereof. The document D is a sheet (sheet) on which a document image for image forming by the above-described image forming device is formed. The scanner 7 scans the document D supported on the document table 6, to read the document image of the document D.

Further, the scanner 7 scans the document D to be fed to a reading position of the document table 6 by an automatic document feeder 110 described later, to read the document image of the document D. Each of the above-described image forming units 10Y, 10M, 10C, 10B of the respective colors forms an electrostatic latent image based on each color component of the document image read by the scanner 7, on the above-described photoreceptor. Each of the image forming units 10Y, 10M, 10C, 10B develops the electrostatic latent image using the toner of each color, to form the above-described toner image of each color, on the above-described photoreceptor. The images formed on the respective photoreceptors are transferred to the transfer belt, as described above.

The sheet P is picked up from the sheet feeding cassette 2 by the pickup roller 2 a, as described above, and is conveyed to the transfer position by the transfer roller 3 of the transfer device, and the fixing position by the fixing device, by the main conveying path 8. As described above, the superposed image of the respective colors carried on the transfer belt 1 is transferred to the sheet P at the above-described transfer position by the transfer roller 3. The sheet P on which the image has been transferred is further made to pass through the fixing unit 4 by the main conveying path 8. The toners of the respective colors of the above-described image which has been transferred to the sheet P are melted by the fixing unit 4, and thereby are fixed to the sheet P. The sheet P formed with the image is further discharged to the sheet discharge tray 5 by the sheet discharge roller 5 a, as described above.

The copying machine 100 further has an inversion conveying path 9. When forming images on both surfaces of the sheet P, the copying machine 100 reverses the sheet discharge roller 5 a in the state to sandwich the sheet P formed with an image on the above-described one surface (a front surface). The sheet discharger roller 5 a is reversed, to lead the relevant sheet P to the inversion conveying path 9. The inversion conveying path 9 feeds the sheet P led by the sheet discharge roller 5 a to the main conveying path 9 again, in the front/back inversed state. The main conveying path 8 conveys the sheet P in the front/back inversed state to the transfer position by the transfer roller 3 of the transfer device, and the fixing position by the fixing device, as described above. Accordingly, an image is also formed on the other surface (a back surface) of the sheet P.

FIG. 2 is a schematic diagram showing the automatic document feeder 110 (hereinafter, referred to as the ADF 110) that is an embodiment of a sheet conveying apparatus. The ADF 110 has a sheet feeding tray 111, a sheet discharge tray 112, and a document conveying device, as shown in FIG. 2. The sheet feeding tray 111 holds the document D. The document D held by the sheet feeding tray 111 is fed to a document conveying path 113 of a document conveying device, by a sheet feeding roller not shown, for example. The document conveying device has the above-described document conveying path 113 (a second conveying path). The document conveying path 113 conveys the document D fed from the sheet feeding tray 111 to the above-described reading position. Further, the document conveying path 113 discharges the document D which has passed through the above-described reading position to the sheet discharge tray 112. The sheet discharge tray 112 holds the discharged document D. Further, the document conveying device of the ADF 110 has a plurality of conveying rollers arranged along the document conveying path 113, and sound deadening structures 20 described later which are arranged in the middle of the document conveying path 13.

In the copying machine 100 having the above-described ADF 110, since the sheet P is conveyed in the chassis 101, and the document D is conveyed in the ADF 110, the above-described operation sound in accompany with the conveying operation is generated. Since the copying machine 100 is generally installed in an office and used therein, it is desirable to make the operation sound like this small as much as possible.

For this reason, in the present embodiment, the sound deadening structures 20 are respectively provided in the main conveying path 8 and the inversion conveying path 9 each of which conveys the sheet P in the chassis 101 of the copying machine 100, and further in the document conveying path 113 which conveys the document D in the ADF 110. By this means, sounds generated at the time of conveying the sheets (the sheet P and the document D) can be reduced, as a whole of the copying machine 100 including the ADF 110. Since the sound deadening structures provided in the respective conveying paths 8, 9, 113, and sound deadening mechanisms thereof are the same, in the following description, the sound deadening structure 20 provided in the document conveying path 113 of the ADF 110 will be described as a representative, and the description of the sound deadening structures 20 provided in the main conveying path 8 and the inversion conveying path 9 will be omitted.

FIG. 3 is an enlarged sectional diagram of the sound deadening structure 20 provided in the ADF 110 of FIG. 2. FIG. 4 is a sectional diagram along F4-F4 of FIG. 2, and FIG. 5 is a sectional diagram along F5-F5 of FIG. 2. In the present embodiment, the sound deadening structures 20 are provided at two positions along the document conveying path 113 (hereinafter, simply referred to as the conveying path 113).

The above-described document conveying device of the ADF 110 has an upper guide plate 22 (a first conveying guide) and a lower guide plate 24 (a second conveying guide). The upper guide plate 22 is arranged in the conveying path 113 at one surface side (for example, an upper side in FIG. 4 and FIG. 5) of the document D to be conveyed in the conveying path 113. The lower guide plate 24 is arranged in the conveying path 113 at the other surface side (for example, a lower side in FIG. 4 and FIG. 5) of the document D to be conveyed in the conveying path 113. In the following description, one surface and the other surface of the document D are sometimes called conveying surfaces. Further, one surface side and the other surface side of the document D are sometimes called one side of the conveying surface and the other side of the conveying surface, respectively. The above-described sound deadening structure 20 has a structure obtained by performing a shape processing of the upper guide plate 22 (first conveying guide), and the lower guide plate 24 (second conveying guide), for example. The upper guide plate 22 and the lower guide plate 24 respectively have flat inner surfaces 22 a, 24 a which are opposite to each other while sandwiching the conveying surfaces of the document D therebetween, as shown in FIG. 4.

Specifically, as the sound deadening structure 20, at least one concave portion 23 (a first concave portion) is provided to be recessed in the direction to separate from the conveying surface of the above-described document D, in the upper guide plate 22. The concave portion 23 has a semi-cylindrical shape. Further, at least one concave portion 25 (a second concave portion) is provided opposite to the concave portion 23, and to be recessed in the direction to separate from the conveying surface of the above-described document D, in the lower guide plate 24. For example, the concave portion 23 is formed so that a part of the upper guide plate 22 is recessed from the inner surface 22 a in the direction to separate from the conveying surface of the document D. For example, the concave portion 25 is formed so that a part of the lower guide plate 24 is recessed from the inner surface 24 a in the direction separate from the conveying surface of the document D. These two concave portions 23, 25 have the same shapes which are plane symmetric to the conveying surface, and are provided in the posture that the respective longitudinal axes thereof are along the conveying direction of the document D. Accordingly, when the two concave portions 23, 25 are faced to each other, a space with an approximately columnar shape is formed in the middle of the conveying path 113. This space functions as a sound absorption chamber 30.

The above-described sound absorption chamber 30 is provided in the middle of the conveying path 113, and thereby the sound deadening structure 20 changes an area of a cross section of the conveying path 113 cut along a plane orthogonal to the conveying direction of the document D, along the conveying direction. The cross-section area of the conveying path 113 is changed in the middle thereof in this manner, to cause an acoustic impedance thereof to be changed, and thereby it is possible to make the conveying path 113 have a sound deadening function.

When a sound pressure of a sound to be propagated through the conveying path 113 is p, a cross-section area of the conveying path is S, and a particle velocity is μ, an acoustic impedance Z can be expressed as,

Z=p/Sμ.

For example, when the cross-section area of the conveying path 113 is increased along the conveying direction of the document D, and thereby the acoustic impedance is changed (is decreased), a part of a sound is reflected (r1, r3) at positions (R1, R3 in FIG. 3) where the acoustic impedance is changed, and is returned in a direction reverse to the conveying direction of the document D. By this means, the sound to be propagated in the conveying direction is decreased, and in addition, the reflected sound and the sound to be propagated in the conveying direction interfere with each other, and thereby the sound to be propagated in the conveying direction attenuates.

In addition, when the cross-section area of the conveying path 113 is decreased, and thereby the acoustic impedance is changed (is increased), a part of a sound is reflected (r2, r4) at positions (R2, R4 in FIG. 3) where the acoustic impedance is changed, and is returned in a direction reverse to the conveying direction of the document D. By this means, the sound to be propagated in the conveying direction is decreased, and in addition, the reflected sound and the sound to be propagated in the conveying direction interfere with each other, and thereby the sound to be propagated in the conveying direction further attenuates.

As described above, the concave portion 23 and the concave portion 25 are respectively provided in the upper guide plate 22 and the lower guide plate 24 defining the conveying path 23, and the sound deadening structure 20 of the present embodiment is formed by performing a shape processing of the upper guide plate 22 and the lower guide plate 24, to cause the cross-section area of the conveying path 113 to be changed, and reduces the sound to be propagated through the conveying path 113. Even if the sound deadening structure 20 is provided at one position on the conveying path, the sound deadening effect can be exerted, but the sound deadening structures 20 are provided at a plurality of separate positions (in the present embodiment, two positions) in the conveying direction, as in the present embodiment, and thereby it is possible to further enhance the sound deadening effect.

In addition, in the present embodiment, two sets of the semi-cylindrical concave portions 23, 25 (sound absorption chambers 30) are provided in the conveying path 113, but the sound absorption chambers 30 may be provided at a plurality of positions in the width direction orthogonal to the conveying direction, as shown in FIG. 6, for example. In addition, as shown in FIG. 7, concave portions 27, 29 each having a rectangular sectional shape are provided opposite to each other, and thereby a sound absorption chamber 40 may be provided in the conveying path 113. Further, as shown in FIG. 8, a plurality of the sound absorption chambers 40 may be provided in the conveying path 113 side by side in the above-described width direction.

In addition, when a plurality of the sound absorption chambers 30 (40) provided along the conveying direction are arranged at the same positions in the width direction, the same part of a leading edge in the conveying direction of the document D to be conveyed comes to pass through the sound absorption chambers 30 (40) many times, and thereby there is a possibility that a trouble such as folding and break is generated at the leading edge of the document D. For the reason, as shown in FIG. 9, for example, so that the same part of the leading edge of the document D does not pass through the sound absorption chambers 30 (40) many times, the sound absorption chambers 30 (40) which are continuously provided in the conveying direction may be arranged so that the positions thereof in the width direction are shifted. In addition, in FIG. 9, the sound absorption chamber 30 is illustrated as a sectional shape seen from the conveying direction, so that the description is easily understandable. In addition, in FIG. 9, the conveying direction of the document D is shown by an arrow T.

Hereinafter, an attenuation amount of a sound by the above-described sound deadening structure 20 will be described using a testing device 50 shown in FIG. 10. The testing device 50 has, in the middle of a sound propagation path 52 having a circular sectional shape (a cross-section area S1), a columnar sound absorption chamber 54 having a circular sectional shape of a cross-section area S2 larger than the cross-section area S1. When a length of the sound absorption chamber 54 along the sound propagation direction (an arrow direction in the drawing) is L, a ratio of the cross-section areas is m (=S2/S1), a wavelength of a propagated sound is λ, and k=2n/λ, an attenuation amount of a sound which is propagated in this device is expressed as follows.

Attenuation amount=10log₁₀{1+¼(m−1/m)²sin² kL}  (dB)

FIG. 11 is a graph showing the relation between the above-described attenuation amount and L/λ. A vertical axis of this graph is an attenuation amount, a horizontal axis is L/λ. The graph of FIG. 11 shows a case in which the cross-section area ratio m=10, and a case in which m=50, in comparison with each other. In this case, it is assumed that a temperature of the propagation path of the sound in the testing device 50 was 30° C.

According to FIG. 11, it is understood that as the cross-section area ratio m of the propagation path is larger, the attenuation amount of the sound becomes larger. That is, in order to enhance the sound deadening effect, it is only necessary to make the cross-section area S2 of the sound absorption chamber 54 larger than the cross-section area S1 of the sound propagation path 52. When the conveying path 113 of the document D is considered in place of it, since a conveying space (a gap between the inner surface 22 a of the upper guide plate 22 and the inner surface 24 a of the lower guide plate 24) of the document D is very narrow, it is easy to make the cross-section area of the conveying path 113 several tens times, by providing the sound absorption chamber 30 (40).

In addition, as can be understood from FIG. 11, when the length L of the sound absorption chamber 54 is λ/2, λ, 3λ/2, 2λ . . . , the attenuation amount of the sound becomes 0, and when L is λ/4, 3λ/4, 5λ/4 . . . , the attenuation amount becomes maximum. That is, the attenuation amount of the sound changes depending on the relation between the length L of the sound absorption chamber 54 and the wavelength λ of the sound. In other words, a plurality of the sound absorption chambers are provided in the propagation path of the sound, and the lengths L of the respective sound absorption chambers are made different, and thereby it is possible to attenuate a sound of a wide frequency band.

FIG. 12 shows an attenuation amount of the sound, when the length L of the sound absorption chamber is set to 0.01 m, and a frequency Hz of the propagated sound and the cross-section area ratio m of the conveying path are used as parameters. According to FIG. 12, it is understood that when the cross-section area ratio m is set to 40, the attenuation amount of a sound of 8000-10000 Hz becomes the largest.

FIG. 13 shows an attenuation amount of the sound, when the length L of the sound absorption chamber is set to 0.02 m, and the frequency Hz of the propagated sound and the cross-section area ratio m of the conveying path are used as parameters. In addition, FIG. 14 shows an attenuation amount of the sound, when the length L of the sound absorption chamber is set to 0.03 m, and the frequency Hz of the propagated sound and the cross-section area ratio m of the conveying path are used as parameters. According to FIG. 13 and FIG. 14, it is understood that when the cross-section area ratio m is set to 40, the attenuation amount of a sound of 5000 Hz becomes the largest.

As described above, the attenuation amount of the sound changes depending on the cross-section area ratio m of the conveying path. In addition, the length L of the sound absorption chamber has an optimum value wherein an attenuation amount can be made the largest in accordance with a frequency of the sound to be attenuated. In other words, a plurality of the sound absorption chambers are provided in the middle of the conveying path of a sheet, and the lengths of the respective sound absorption chambers are made different, and thereby it is possible to reduce a sound of a wide frequency band.

FIG. 15 is a graph showing change of an attenuation amount of the sound when the frequency Hz of the propagated sound and the length L of the sound absorption chamber are used as parameters. A vertical axis of this graph is an attenuation amount, a horizontal axis is a length of the sound absorption chamber. At this time, the temperature of the conveying path was set to 30° C., and the cross-section area ratio m was set to 10. According to this, when L is changed, a sound of each frequency has at least one peak of the attenuation amount, and it can be understood that an optimum value of a length of the sound absorption chamber exists.

In a case of a sound of 1000 Hz, for example, an attenuation amount becomes the largest, when the length L of the sound absorption chamber is set to 0.087 m. In addition, in a case of a sound of 5000 Hz, for example, an attenuation amount becomes the largest, when the length L of the sound absorption chamber is set to 0.017 m, 0.053 m, 0.087 m, 0.122 m. In addition, in a case of a sound of 10000 Hz, for example, an attenuation amount becomes the largest, when the length L of the sound absorption chamber is set to 0.009 m, 0.026 m, 0.044 m, 0.061 m, 0.079 m, 0.096 m, 0.114 m, 0.131 m.

As described above, according to the present embodiment, the cross-section area of the conveying path is changed in the middle of the conveying path of a sheet, and thereby an acoustic impedance is changed before and after the cross-section area is changed, and accordingly it is possible to reduce the sound which is propagated through the conveying path. In addition, at this time, a plurality of the sound absorption chambers with different lengths, along the conveying direction, of the sound absorption chambers for changing the cross-section area of the conveying path are provided, and thereby it is possible to reduce a sound of a wide frequency band. In this case, it is not necessary to provide a sound absorbing material on the conveying guide unlike a conventional one, and thereby the number of components can be reduced, and the manufacturing cost of the apparatus can be reduced. That is, according to the present embodiment, it is possible to provide a sheet conveying apparatus which can reduce a sound of a wider frequency band with an inexpensive configuration, and an image forming apparatus having this apparatus.

While certain embodiments have been described, these embodiments have been presented by way of example only. For example, in the above-described embodiment, the case that the concave portions are respectively provided in the guide plates (the upper guide plate 22 and the lower guide plate 25) at the both sides of the conveying surface of a sheet, and thereby the sound absorption chamber is provided in the middle of the conveying path has been described. But the sound absorption chamber is not limited to this, but the concave portion 23 is provided only in the upper guide plate 22, and thereby a sound absorption chamber may be provided between the concave portion 23 and the lower guide plate 24. That is, a shape and a size of the sound absorption chamber can be designed optionally, and it is only necessary that a plurality of sound absorption chambers with different lengths along the conveying direction are provided. In addition, the concave portions may not be the same shapes that are plane symmetrical with respect to the conveying surface.

Further, a plurality of protrusions 60 may be provided on the inner surface 22 a of the upper guide plate 22, as shown in FIG. 16. A plurality of these protrusions 60 function so as to reduce an area where a sheet to be conveyed contacts the inner surface 22 a of the upper guide plate 22. By this means, it is possible to reduce a friction sound due to sliding contact between the sheet and the upper guide plate 22. In addition, a plurality of these protrusions 60 may be an elongated rib continuous in the conveying direction.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A sheet conveying apparatus, comprising: a first conveying guide which is arranged at one surface side of a sheet to be conveyed; a second conveying guide which is arranged opposite to the first conveying guide, and at the other surface side of the sheet; and at least one first concave portion which is provided in the first conveying guide to be recessed in a direction to separate from the one surface of the sheet.
 2. The sheet conveying apparatus according to claim 1, further comprising: at least one second concave portion which is provided opposite to at least the one first concave portion, and in the second conveying guide to be recessed in a direction to separate from the other surface of the sheet.
 3. The sheet conveying apparatus according to claim 2, wherein: a plurality of sets of the facing first and second concave portions are provided.
 4. The sheet conveying apparatus according to claim 2, further comprising: a plurality of sound absorption chambers each of which is formed by the facing first and second concave portions.
 5. The sheet conveying apparatus according to claim 4, wherein: the plurality of sound absorption chambers have different lengths in a conveying direction of the sheet.
 6. The sheet conveying apparatus according to claim 5, wherein: the first conveying guide has an inner surface which is opposite to the one surface of the sheet, and a protrusion which is provided on the inner surface to contact the sheet.
 7. An image forming apparatus, comprising: an image forming device which forms an image on a sheet; a first conveying path which conveys a sheet on which the image is to be formed to the image forming device, and discharges the sheet which has passed through the image forming device; a second conveying path which conveys a sheet formed with a document image for forming the image to a reading position for reading the document image, and discharges the sheet from the reading position; a first conveying guide which is provided in at least one conveying path of the first and second conveying paths, and is arranged at one surface side of the sheet; a second conveying guide which is provided in the one conveying path, and is arranged opposite to the first conveying guide, and at the other surface side of the sheet; and at least one first concave portion which is provided in the first conveying guide to be recessed in a direction to separate from the one surface of the sheet.
 8. The image forming apparatus according to claim 7, further comprising: at least one second concave portion which is provided opposite to at least the one first concave portion, and in the second conveying guide to be recessed in a direction to separate from the other surface of the sheet.
 9. The image forming apparatus according to claim 8, wherein: a plurality of sets of the facing first and second concave portions are provided.
 10. The image forming apparatus according to claim 9, further comprising: a plurality of sound absorption chambers each of which is formed by the facing first and second concave portions.
 11. The image forming apparatus according to claim 10, wherein: the plurality of sound absorption chambers have different lengths in a conveying direction of the sheet.
 12. The image forming apparatus according to claim 11, wherein: the first conveying guide has an inner surface which is opposite to the one surface of the sheet, and a protrusion which is provided on the inner surface to contact the sheet. 